Lubricating oil used in gasoline engines for lubrication of moving components, hereinafter simply referred to as “oil”, deteriorates by the depletion of the additives and the increase in the acidity of the oil, as measured by a quantity called the total acid number (TAN). Oil in Diesel engines is degraded by the same mechanisms as in gasoline engines, but with the additional presence of soot particles, which increases as the oil ages. During usage of a Diesel engine, the crankcase oil gradually builds up soot which is a combustion product in the combustion chamber of the engine and which is transferred in small amounts to the crankcase oil. When the soot builds up to an unacceptable amount, say about four percent by mass or weight of the oil, the lubricating quality of the oil is inhibited. Thus, it is necessary to change the crankcase oil whenever the soot content reaches an unacceptable value. For this purpose, it is desirable to measure the soot content in the crankcase oil in order to detect the presence of an unacceptable percentage of soot.
Many different methods or techniques have been proposed for the measurement of soot in Diesel engine oil. In order to make the soot measurement on operating vehicles, it is necessary to provide a measuring system which is sufficiently inexpensive to incorporate on automotive vehicles made in large numbers and sufficiently rugged to withstand the Diesel engine operating environment. Moreover, a method of measuring soot in crankcase oil must be valid for many types of oil, both natural and synthetic, and containing many different types of additives.
U.S. Pat. Nos. 5,824,889; 5,656,767; and 4,733,556, and the Society of Automotive Engineers technical papers 970847 and 910497 describe how the dielectric constant (permittivity) can be used to describe the condition of Diesel oil or to detect the presence of moisture and antifreeze. The dielectric constant varies with the total acid number as well as the soot concentration; however, the variation in the dielectric constant between fresh oil and contaminated oil is not great. Furthermore, the dielectric constant of oil is also influenced by the temperature of the oil, by the specific formulation of a given brand of oil due to the additives that are deliberately added to the oil, by antifreeze, and by water.
The prior art describes a number of techniques that measure the dielectric constant with a sensor built like a capacitor. The capacitor like sensor includes two metal electrodes with the lubricating oil acting as the dielectric between the electrodes. The two metal electrodes take the form of two parallel plates or two concentric cylinders. Most of these sensors determine the permittivity of the oil through a measurement of the capacitance between the metal electrodes.
Sensors that measure the loss tangent, essentially the ratio of the electrical conductivity of the oil to the dielectric constant, have also been proposed. These sensors assume that the dielectric losses can be described by a single relaxation method, which is an inapplicable assumption, as used oil contains conductive particles (soot), polar molecules (water or antifreeze), and charged particles (ions). The presence of polar compounds increases the dielectric constant and affects the loss tangent making the loss tangent more sensitive to parasitic signals, such as the brand of oil or the presence of water and antifreeze. Delphi Automotive Systems has shown experimentally that such a sensor works only for selected oils due to the dependence of the measurement on the dielectric constant.
Delphi Automotive Systems also possesses a design for a gasoline engine oil contaminant sensor that measures the electrical conductivity of the oil using D.C. or a low frequency (below 1 kHz). The sensor consists of two metal electrodes, which can be parallel plates or concentric cylinders or rings. The conductivity is determined through a measurement of the electrical resistance between the electrodes. This sensor mainly detects the changes in the concentration of ions in the oil. In this regard, fresh oil is slightly basic. As the oil ages, the combustion products create acidic ions in the oil. At first, the acids neutralize the bases and the conductivity decreases. As the oil ages further, the increase in acidic ions makes the conductivity rise again. This makes for a very good oil quality sensor in gasoline engines. However, the soot in Diesel engine oils masks the ion density changes and renders this type of sensor useless.
Accordingly, what is needed in the art is a more robust method of detecting soot in Diesel engine oils, which is independent of the brand of oil and immune to the effects of adding fresh oil with different dielectric or electrical properties than the original oil.